Peptides are a diverse class of molecules having a variety of important chemical and biological properties. Some examples include; hormones, cytokines, immunoregulators, peptide-based enzyme inhibitors, vaccine antigens, adhesions, receptor binding domains, enzyme inhibitors and the like. The cost of chemical synthesis limits the potential applications of synthetic peptides for many useful purposes such as large scale therapeutic drug or vaccine synthesis. There is a need for inexpensive and rapid synthesis of milligram and larger quantities of naturally-occurring polypeptides. Towards this goal many animal and bacterial viruses have been successfully used as peptide carriers.
The safe and inexpensive culture of plants provides an improved alternative host for the cost-effective production of such peptides. During the last decade, considerable progress has been made in expressing foreign genes in plants. Foreign proteins are now routinely produced in many plant species for modification of the plant or for production of proteins for use after extraction. Animal proteins have been effectively produced in plants (reviewed in Krebbers et al., 1992).
Vectors for the genetic manipulation of plants have been derived from several naturally occurring plant viruses, including TMV (tobacco mosaic virus). TMV is the type member of the tobamovirus group. TMV has straight tubular virions of approximately 300×18 nm with a 4 nm-diameter hollow canal, consisting of approximately 2000 units of a single capsid protein wound helically around a single RNA molecule. Virion particles are 95% protein and 5% RNA by weight. The genome of TMV is composed of a single-stranded RNA of 6395 nucleotides containing five large ORFs. Expression of each gene is regulated independently. The virion RNA serves as the messenger RNA (mRNA) for the 5′ genes, encoding the 126 kDa replicase subunit and the overlapping 183 kDa replicase subunit that is produced by read through of an amber stop codon approximately 5% of the time. Expression of the internal genes is controlled by different promoters on the minus-sense RNA that direct synthesis of 3′-coterminal subgenomic mRNAs which are produced during replication (FIG. 1). A detailed description of tobamovirus gene expression and life cycle can be found, among other places, in Dawson and Lehto, Advances in Virus Research 38:307–342 (1991). It is of interest to provide new and improved vectors for the genetic manipulation of plants.
For production of specific proteins, transient expression of foreign genes in plants using virus-based vectors has several advantages. Products of plant viruses are among the highest produced proteins in plants. Often a viral gene product is the major protein produced in plant cells during virus replication. Many viruses are able to quickly move from an initial infection site to almost all cells of the plant. Because of these reasons, plant viruses have been developed into efficient transient expression vectors for foreign genes in plants. Viruses of multicellular plants are relatively small, probably due to the size limitation in the pathways that allow viruses to move to adjacent cells in the systemic infection of entire plants. Most plant viruses have single-stranded RNA genomes of less than 10 kb. Genetically altered plant viruses provide one efficient means of transfecting plants with genes coding for peptide carrier fusions.